Editing Lossy compression (section)

===Editing of lossy files===
{{see also|commons:Commons:Software#JPEG|commons:Commons:Software#Ogg Vorbis (audio)}}
By modifying the compressed data directly without decoding and re-encoding, some editing of lossily compressed files without degradation of quality is possible. Editing which reduces the file size as if it had been compressed to a greater degree, but without more loss than this, is sometimes also possible.

====JPEG====
The primary programs for lossless editing of JPEGs are <code>[[jpegtran]]</code>, and the derived <code>exiftran</code> (which also preserves [[Exif]] information), and [http://sylvana.net/jpegcrop/ Jpegcrop] (which provides a Windows interface).

These allow the image to be [[Cropping (image)|cropped]], rotated, [[flipped image|flipped]], and [[flopped image|flopped]], or even converted to [[grayscale]] (by dropping the [[chrominance]] channel). While unwanted information is destroyed, the quality of the remaining portion is unchanged.

Some other transforms are possible to some extent, such as joining images with the same encoding (composing side by side, as on a grid) or pasting images such as logos onto existing images (both via [http://sylvana.net/jpegcrop/jpegjoin/ Jpegjoin]), or scaling.<ref>{{Cite web|url=http://sylvana.net/jpegcrop/jpegtran/|title=New jpegtran features|website=sylvana.net|access-date=2019-09-20}}</ref>

Some changes can be made to the compression without re-encoding:
* Optimizing the compression (to reduce size without change to the decoded image)
* Converting between progressive and non-progressive encoding.

The freeware Windows-only [[IrfanView]] has some lossless JPEG operations in its <code>JPG_TRANSFORM</code> [[Plug-in (computing)|plugin]].

====Metadata====
Metadata, such as [[ID3 tag]]s, [[Vorbis comment]]s, or [[Exif]] information, can usually be modified or removed without modifying the underlying data.

====Downsampling/compressed representation scalability====
One may wish to [[downsample]] or otherwise decrease the resolution of the represented source signal and the quantity of data used for its compressed representation without re-encoding, as in [[bitrate peeling]], but this functionality is not supported in all designs, as not all codecs encode data in a form that allows less important detail to simply be dropped. Some well-known designs that have this capability include [[JPEG 2000]] for still images and [[H.264/MPEG-4 AVC]] based [[Scalable Video Coding]] for video.  Such schemes have also been standardized for older designs as well, such as [[JPEG]] images with progressive encoding, and [[MPEG-2]] and [[MPEG-4 Part 2]] video, although those prior schemes had limited success in terms of adoption into real-world common usage. Without this capacity, which is often the case in practice, to produce a representation with lower resolution or lower fidelity than a given one, one needs to start with the original source signal and encode, or start with a compressed representation and then decompress and re-encode it ([[transcoding]]), though the latter tends to cause [[digital generation loss]].

Another approach is to encode the original signal at several different bitrates, and then either choose which to use (as when streaming over the internet – as in [[RealNetworks]]' "[[SureStream]]" – or offering varying downloads, as at Apple's [[iTunes Store]]), or broadcast several, where the best that is successfully received is used, as in various implementations of [[hierarchical modulation]]. Similar techniques are used in [[mipmap]]s, [[pyramid (image processing)|pyramid representation]]s, and more sophisticated [[scale space]] methods. Some audio formats feature a combination of a lossy format and a lossless correction which when combined reproduce the original signal; the correction can be stripped, leaving a smaller, lossily compressed, file. Such formats include [[MPEG-4 SLS]] (Scalable to Lossless), [[WavPack]], [[OptimFROG DualStream]], and [[DTS-HD Master Audio|DTS-HD Master Audio in lossless (XLL) mode]]).